In the latest Zotero beta, we've added a Recently Read collection to the top of the collections list. This is a virtual collection that shows the items in a library with attachments you've opened in the last two weeks, most recent first. It includes a Last Read column, which you can also add to other views if you'd like to.

Opening an attachment or changing pages will bump the item to the top of the list. (Changing pages will only update its last-read time once every few minutes.)

This data syncs between devices, so you can quickly find a file on another device that you've read elsewhere. We'll be adding this view to the mobile apps soon.